Even though the standard model of the strong and electroweak interactions has proven enormously successful, it need not be the case that a single Higgs-doublet field is responsible for giving masses to the weakly interacting vector bosons and the fermions. In this paper we explore the phenomenology of a Higgs sector for the standard model which contains both doublet and triplet fields [under SU(2)L]. The resulting Higgs bosons have many exotic features and surprising experimental signatures. Since a critical task of future accelerators will be to either discover or establish the nonexistence of Higgs bosons with mass below the TeV scale, it will be important to keep in mind the alternative possibilities characteristic of this and other nonminimal Higgs sectors. 1673It is useful to consider the transformation of the 4 and x fields under SU(2), XSU(2), , 4--. u,~u;, and tx-+ ULxUR, where UL,, = exp( -ieLXR^nL,, .TL,, 1, and the TL,, generators are represented as specified above. The SU(2)L and U(1) invariances of the standard model are to be associated with TL and T i , respectively. In particular, note that the U(1) hypercharge associated with the B field is represented by right multiplication by the appropriate T; matrix (so that Q = T; + T; ). The full SU(2), group will be associated with the custodial symmetry required to have p= 1. In particular, tree-level invariance for the gauge-boson-mass terms under the custodial SU(2), is arranged by giving the x0 and lo the same vacuum expectation value. [However, since the hypercharge interaction with the B field breaks the custodial SU(2), , there are potentially infinite contributions to p-1 at one loop. We shall return to this issue later.1 We define ( x O ) = ( $ ) = b , and also take (4°)=n/2/2. It will be convenient to use the notation where CH and sH are the cosine and sine of a doublettriplet mixing angle. We will also employ the subsidiary field for the complex neutral and charged fields, respectively. The W' and Z are given mass by absorbing the Goldstone bosons G : =cH4'+sH$*, ~! = i ( -c~~~~+ s~x~~). (2.6)The gauge-boson masses so obtained are A is a 2 X 2 representation of the Y = 2 complex triplet field,

Eective Lagrangians can be used to parametrize the eects of physics beyond the standard model. Assuming the complete theory is a gauge theory, w e determine which eective operators may be generated at tree level, and which are only generated at loop level. The latter are be suppressed by factors of 1=16 2 and will therefore be quite dicult to detect. In particular, all operators changing the Standard-Model structure of the triple-gauge-vector couplings fall into this category. W e also point out that in certain cases, dimension-eight operators may be more important than dimension-six operators. We discuss both the linear and non-linear representation of the Higgs sector.

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Extensions of the Higgs sector of the standard model (SM) that employ only doublet and singlet Higgs-field representations are not the only ones that guarantee p= l at the tree level. Higgs sectors containing triplet (and higher) representations can be constructed in such a way that there is a treelevel custodial SU(2) symmetry yielding p = I. However, this custodial SU(2) is inevitably violated at the one-loop level. We explore the implications of this violation in the context of a Higgs sector containing triplet fields. In particular, we show that it leads to one-loop corrections t o p and to certain mixings among the Higgs bosons and gauge bosons of the model that are quadratically divergent, thereby creating a new naturalness problem for p and for certain Higgs-boson couplings. This new class of naturalness problems first arises for a Higgs sector with triplet representations. making such a sector an interesting case study. A priori, deviations from p= 1 induced at one loop are of arbitrary magnitude and sign. We demonstrate that the fine-tuning required to keep one-loop corrections to p and the Higgs-boson couplings small is similar in nature to that required in the SM to keep the SM Higgs-boson mass in the perturbative regime.

We consider the most general dimension 5 effective Lagrangian that can be built using only Standard Model fields plus right-handed neutrinos, and find that there exists a term that provides electroweak moments (i.e., couplings to the 𝑍 and photon) for the right-handed neutrinos. Such term has not been described previously in the literature. We discuss its phenomenology and the bounds that can be derived from LEP results and from the observation of the cooling process of red giants and supernovae. MotivationNeutrino physics has been a hot topic of research and discussion in the last thirty years, and especially since we have compelling evidence that the structure of their masses is highly nontrivial (for a review on the subject, see [1,2]). The remarkable smallness of these masses, at least a factor 10 5 lighter than the electron mass, is usually regarded as an indication that new physics should be involved in their generation.The definite nature of this new physics, of course, depends on the specific model one wishes to consider, and there are plenty of them: the seesaw mechanism [2], which is one of the most popular proposals, the several models for radiative generation of masses [3] and many more. Precisely this abundance of proposals makes appealing the possibility of studying the new physics associated to neutrinos in a model-independent way. This can be done most easily if the new particles are very heavy, by eliminating them from our description of low-energy physics; the result is called an effective theory 1 , and has been for long a powerful tool for examining neutrino physics (see, for example, two early but interesting applications in [5,6]).However, in our current situation, and given the present knowledge and unknowns about neutrino masses, a piece of the puzzle has been mainly ignored: as we don't know if the neutrino masses are Dirac or Majorana, right-handed neutrinos might be among the light degrees of freedom of the theory. If they are, they must be included in the effective theory, or otherwise it will be incomplete. In any case, we don't know yet the nature of neutrino masses, so it seems sensible to include them for the sake of generality. This constitutes our starting point: we want to inspect an effective theory which can describe all possible neutrino mass structures, and see what insight it can cast upon new physics effects;1 For an example of the procedure, see, for instance, [4].